The present invention relates to a process for the production of pure, distilled isocyanates and to a multi-stage distillation process for working-up the residue of this production process.
Processes for the large-scale production of distilled isocyanates by reacting an amine with phosgene in a solvent are known. Such processes are described in detail in the literature (See, e.g., Ullmanns Encyklopadie der technischen Chemie Fourth Edition, Volume 13, pages 347-357, Verlag Chemie GmbH, D-69469 Weinheim, 1977). It is also known that a by-product stream is generated during the production of pure isocyanates by such distillation processes. This by-product stream has to be disposed of as a residue after free isocyanate has been separated as much as possible by distillation. In laboratory tests, it is possible to distill off significant additional amounts of free isocyanate from this residue stream. However, the remaining residue becomes a hard cross-linked mass which cannot be handled in a commercial process. Therefore, about 20-40% of free isocyanate is left in the residue stream from the commercial process to ensure that the residue stream can be handled. Consequently, valuable material is lost and the amount of waste to be disposed of is greater.
Processes for recovering additional free isocyanate from the residue are known. For example, GB 1,408,745 describes an extraction process for recovering the free isocyanate. EP 269,218 describes a process in which the residue stream is heated using a bath of molten metal or metal salts. A distillation process performed on isocyanate-containing residue in a fluidized bed is described in DE 2,915,830. Each of these processes requires costly apparatus and/or auxiliary process materials. Auxiliary process materials such as metals or metal salts, for example, make subsequent disposal considerably more difficult.